Note: Descriptions are shown in the official language in which they were submitted.
1 337555
Background of the Invention:
Field of the Invention:
The present invention relates to coated steel
sheets or strips (herein called steel sheets) having
a coating which can provide excellent corrosion
resistance, particularly corrosion resistance in their
worked portions, and useful for applications in
automobiles, home electric appliances, and constructions.
Description of Related Arts:
Galvanized steel sheets have long been used
widely as a surface treated steel sheet, because they
can be commercially produced on a mass-production scale
without sacrificing their corrosion resistance before
or after paint coating and workability as well as strength
inherent to cold rolled steel sheets.
In more recent years, trials have been made
using on galvanized steel sheets as rust preventive steel
sheets in automobiles, particularly in cold regions,
20 for preventing the rust caused by salt dispersed on
highway roads to prevent the freezing of the roads.
The trend is, however, that more and more demands are
being made for satisfactory corrosion resistance of the
galvanized steel sheets under severe corrosive
environments.
As the means for meeting with the demand for
improved corrosion resistance of the galvanized steel
sheets, it has been known and commonly practiced to
;. ~
~ - 2 - ~
~ . . ,. , ~,
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increase the amount of the zinc coating itself and in
addition to the increased zinc coating amount, many
various proposals have been made for allow coatings
as a means of inhibiting the dissolution of the zinc
coating itself. Most of these proposed alloy
coatings contain iron-group metals such as Fe, Ni and
Co as the alloying element.
The steel sheets electro-plated with a
zinc-iron-group-metal coating as disclosed, for
example, in Japanese Patent Publications 50-29821 and
57-61831 published September 26, 1975 and December
27, 1982, respectively, are characterized by their
excellent corrosion resistance before and after paint
coating and have been successful in commercial
production and application. However, a strong demand
still exists for further improvement of their
corrosion resistance.
Further, steel sheets coated with a zinc
or zinc alloy coating containing chromium have been
proposed as disclosed in Japanese Patent Publications
59-38313 and 59-40234 published September 14, 1984
and September 28, 1984, respectively, Japanese
Laid-Open Patent Applications 61-130498, 61-270398
and 62-54099 laid open on June 18, 1986, November 9,
1986 and March 9, 1987, respectively.
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All of these proposed chromium-containing
coatings contain a very small amount of chromium,
which produces only an auxiliary effect on corrosion
resistance. Therefore, for the purpose of definitely
improving the corrosion resistance, it has long been
desired to precipitate a larger amount of chromium in
the coating.
For applications in automobiles and home
electric appliances, severer demands are being made
for
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improved surface appearance in addition to corrosion
resistance, and to meet with the demands, an appropriate
treatment for the surface brightening of coatings is
strongly desired.
Up to now, no successful art is known for
increasing the chromium content in the zinc coating and
no successful art is known for producing Zn-Cr coatings
wlth a high-chromium content and yet excellent in the
surface appearance.
Thus it is impossible to obtain a satisfactory
coating having good surface brightness and workability
merely by increasing the Cr3 ion concentration in the
plating bath. The increased chromium ion concentration
causes various hindrances in the operation, such as a
sharp lowering in the current efficiency, which prohibit
a commercial production of steel sheets having a
high-chromium alloy coating.
Meanwhile, to meet with the demands for improved
corrosion resistance for applications in automobiles,
in particular, a complex coated steel sheet has been
developed by subjecting the metallic primer coating to
a chromating treatment and then applying an organic
coating thereon.
In the art of these steel sheets having the
complex coating, main considerations have been given
to improvements of the paint composition which forms
the upper-most organic coating, and inevitably, no full
satisfaction has been achieved concerning improvements
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of corrosion resistance, press formability, spot
weldability, etc. For example, a zinc-rich paint
coated steel sheet shows inferior press formability
and is not satisfactory with respect to corrosion
resistance and weldability, and a steel sheet coated
with a paint containing electric conductive pigments
is not wholly satisfactory despite its improved press
formability and weldability. Further, in all types
of the organic coated steel sheets mentioned above,
the organic coating is applied in a relatively large
quantity exceeding 5 ,u in thickness which are rather
detrimental to press formability and weldability.
More recently, steel sheets having a
metallic-organic complex coating also has been
developed in which the organic coating is applied in
a relatively small quantity of less than 5 ,u in
thickness. For these thin-type organic coatings,
trials have been made to improve the corrosion
resistance by introducing rust preventive pigments in
the organic coatings. For example, Japanese
Laid-Open Patent Application 59-162278 of September
13, 1984 discloses an organic coating in which
chromium compounds as the rust preventive pigments
are added to a water-dispersion type emulsion resin,
and Japanese Laid-Open Patent Application 60-50181 of
.....
_ 1 3375~5
March 19, 1985, discloses an organic coating in which
silica is added as the rust preventive pigment.
However, these trials have been found unsuccessful in
improving the corrosion resistance.
The corrosion resistance of the steel
sheets
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having a metallic-organic complex coating mentioned above
is imparted mainly by the organic coating. However,
as the thickness of the organic coatings is required
to be thinner from the points of press formability and
weldability, improvement of the metallic primer coating
itself is required from the point of corrosion resistance.
More specifically, as the metallic primer
coating, Zn coating, Zn-Ni coating, Zn-Fe coating and
the like are applied, and in the thin-type organic
coatings, the organic coating is further thinned when
subjected to the press forming and slight damage or
scratches can easily penetrate the thinned organic coating
to reach the metallic primer coating or even to the
substrate steel sheet, thus causing local exposure of
the metallic coating of the substrate sheet. Therefore,
the corrosion resistance of these coated sheets must
more and more rely on the corrosion resistance of the
metallic primer coating alone. However, the conventional
metallic coatings cannot impart satisfactory corrosion
resistance, and is not reliable for maintaining good
corrosion resistance after press forming.
Summary of the Invention:
Therefore the present invention provides a
coated steel sheet free from the problems of the prior
arts, excellent in corrosion resistance, particularly
corrosion resistance after press forming, and further
excellent in workability, weldability and surface
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brightness.
The present invention is based on the discovery
that it is possible to achieve a markedly enhanced
chromium content in the alloy coating, which has never
been conventionally achieved, if a water soluble cation
polymer is added to a plating bath containing zn2 and
Cr3 which promotes the precipitation of Cr, and that
the resultant coating has satisfactory workability due
to the co-precipitation of a very small amount of the
cation polymer in the coating.
Further, the present invention is based on
the discovery that addition of iron-group metal or metals,
Fe, Ni and Co, to the coating will improve the spot
weldability as required for the applications in
automobiles and home electric appliances.
Still further, the present invention is based
on the discovery made by further studies by the present
inventors that chromium when present with zinc will not
be passivated, but tends to take part in a sacrificial
rust prevention together with zinc, and the corrosion
product of chromium accumulates and forms a
hard-to-dissolve protective film, thereby preventing
a further progress of corrosion. This phenomenon is
considered to provide a high degree of corrosion
resistance.
However, under an exposing condition similar
to the actual condition of service, the hard-to-dissolve
protective film formed by the corrosion product of
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chromium is very likely to be cracked due to a long time
of drying. Therefore, after the lapse of a certain period
of time, corrosion can sharply progress. It is found
by the present inventors that it is effective to
co-precipitate fine particles of oxides in the coating
for preventing such sharp development of corrosion.
The fine particles of oxides intrude into the corrosion
products of zinc and chromium to be strongly combined
therewith. The corrosion products thus combined with
the oxides can still provide good humidity absorbing
ability so that the protective film formed with the
corrosion products is hardly strained even under the
dry condition, resulting in effective prevention of
cracking occurring in the protective film.
Thus on the basis of the above discovery, it
is possible to further stabilize the protective film
formed by the corrosion products of the Zn-Cr alloy
coatings by the addition of fine particles of oxides
in the coating.
Therefore, a further object of the present
invention is to provide a coated steel sheet having a
primer Zn-Cr coating, an intermediate chromate film,
and an upper-most organic coating, which shows excellent
corrosion resistance particularly at worked portions,
and excellent workability and weldability as well. For
this object the Zn-Cr coating contains fine particles
of oxide co-precipitated therewith so as to produce
corrosion products which are very effective to protect
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-
the substrate sheet in event corrosion should occur
locally at paint-coating-defective portions or worked
portions.
A still further object of the present invention
is to provide a process for consistently producing the
coated steel sheets having a high-chromium alloy coating
excellent in corrosion resistance and surface brightness.
For this object, a water soluble cation polymer is
introduced into the Zn-Cr electro-plating bath to promote
the precipitation of chromium, and the ratio of Cr6 ions
to Cr ions in the bath is maintained below a certain
constant value by the anodic oxidation of Cr3 .
Hereinbelow the coated steel sheets and the
process for producing the same according to the present
invention will be summarized.
(1) Electro-plated steel sheets having a primer coating
composed of 5 to 30 % by weight of Cr, 0.005 to 5 % by
weight of cation polymer, with the balance being Zn,
with or without a further Zn or Zn-alloy coating formed
on the primer coating.
(2) Electro-plated steel sheets having a primer coating
composed of no less than 5 % by weight of Cr, no less
than 1 % by weight of iron-group metal, with the total
amount of Cr and the iron-group metal being not more
than 30 % by weight, 0.005 to 5 % by weight of cation
polymer with the balance being Zn, with or without a
further Zn of Zn-alloy coating formed on the primer
coating.
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(3) Electro-plated steel sheets having a primer coating
composed of 5 to 30 % by weight of Cr, 0.1 to 10 % by
weight of fine particles of oxide, and 0.005 to 5 % by
weight of cation polymer with the balance being Zn.
(4) Electro-plated steel sheets having a primer coating
composed of 5 to 30 % by weight of Cr, 0.1 to 10 % by
weight of fine particles of oxide, 0.005 to 5 % by weight
of cation polymer, and 1 to 10 % by weight of iron-group
metal with the balance being Zn.
(5) Electro-plated steel sheets according to (3) or (4)
which further comprise a Zn or Zn-alloy coating formed
on the primer coating.
(6) Coated steel sheets having a primer coating composed
of 5 to 30 % by weight of Cr, 0.1 to 10 % by weight of
fine particles of oxide, 0.005 to 5 % by weight of cation
polymer, with the balance being Zn, a chromate film formed
on the primer coating in an amount of 10 to 150 mg/m2
in total chromium, and an organic coating formed on the
chromate film in 0.3 to 3 ~ thickness.
(7) Electro-plated steel sheets-according to (6), in
which the primer coating further contains 1 to 10 % by
weight of an iron-group metal.
(8) Electro-plated steel sheets having a primer coating
composed of 5 to 30 % by weight of Cr, 0.005 to 5 % by
weight of cation polymer, with the balance being Zn,
a chromate film formed on the primer coating in an amount
of 10 to 150 mg/m2 in total chromium,- and an organic
coating formed on the chromate film in 0.3 to
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3 ~ thickness.
(9) Electro-plated steel sheets having a primer coating
composed of Cr and an iron-group metal in a total amount
not more than 30 % by weight, with Cr being in an amount
not less than 5 % by weight, and the iron-group metal
being in a range from 1/10 to 1/2 of the Cr content,
0.005 to 5 % by weight of cation polymer, with the balance
being Zn, a chromate film formed on the primer coating
in an amount of 10 to 150 mg/m in total chromium, and
an organic coating formed on the chromate film in an
amount of 0.3 to 3 ~.
(10) Electro-plated steel sheets according to any of
(1) to (9) in which the cation polymer is a quaternary
amine polymer.
The process for producing the coated steel
sheets according to the present invention will be
summarized below.
(1) Process for producing an electro-plated steel sheet
having excellent corrosion resistance and surface
brightness, comprising performing electro-plating in
an acidic Zn plating bath containing Cr ions and cation
polymer, with the ratio of Cr6 ions/Cr3 ions being
not more than 0.1.
(2) Process according to (1), in which the acidic bath
contains at least one selected from the group consisting
f Ni2+ F 2+ d Co2+
(3) Process according to (1), in which the acidic bath
further contains fine particles of oxide.
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(4) Process according to any of (1) to (3), in which
the cation polymer is a quaternary amine polymer.
Detailed Description of the Invention:
The excellent corrosion resistance of the
electro-plated steel sheets according to the present
invention is imparted mainly by the chromium content
in the primer coating. For this purpose, S to 30 % by
weight of Cr in the coating is desirable. With the
Cr content less than 5 % by weight in the coating, the
tendency of red rust formation is still present and the
resultant corrosion resistance is not sufficient, although
some improvement is obtained. While with the Cr content
not less than 5 %, the red rust formation during a salt
spray test, for example, can be inhibited and marked
improvement can be obtained.
The high degree of corrosion resistance imparted
by the coated steel sheets according to the present
invention have never been achieved by the conventional
Zn coating, or Zn-alloy coatings, such as Zn-Fe and Zn-Ni
coatings.
As mentioned hereinbefore, Cr when co-present
with Zn, is not passivated, but takes part in the
sacrificial rust prevention together with Zn, and moreover
the corrosion product of Cr precipitates and accumulates
as hard-to-dissolve protective film at corroded portions,
thus prohibiting the progress of corrosion and high
corrosion resistance is assured.
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With the Cr content more than 30 % by weight,
on the other hand, although the resultant corrosion
resistance is satisfactory, the problem of so-called
powdering that the coating strips off during the working
such as press forming cannot be prevented despite the
advantageous effect by the co-precipitation of the cation
polymer as hereinafter described, and therefore such
a high chromium content is not practically applied.
For satisfying both the corrosion resistance
and the workability, 5 to 20 % by weight of Cr content
is more desirable.
The cation polymer used in the present invention
is discovered to be effective to promote the precipitation
of Cr during the electro-plating and also co-precipitates
with Cr in the coating in a very small amount, thus
improving the anti-powdering property. This advantageous
effect by the co-precipitation of the cation polymer
is assumed to derive from the fact that the
co-precipitation prevents the Cr ions from hindering
a uniform electro-deposition and growth of Zn and
iron-group metals so that a uniform and smoothly coated
structure is assured. Thus with the co-precipitation
of the cation polymer in the coating, it is possible
to obtain a dense coating in which Zn and Cr or Zn, Cr
and the iron-group metals are uniformly mixed or alloyed.
For this purpose, 0.005 to 5 % by weight of cation polymer
contained in the coating is desirable. Less than 0.005 %,
no tangible effect is obtained for improving the
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anti-powdering property. On the other hand, the cation
polymer content more 5 ~ in the coating is difficult
to obtain even if the cation polymer concentration in
the plating bath is increased, and an excessive cation
polymer content in the coating tends to lower the coating
adhesion. From the point of working alone, it is enough
if the cation polymer is co-precipitated in the coating
in an amount not less than 1/1000 of the Cr content in
the coating.
Among the water soluble cation polymers used
in the present invention, polymers of quaternary amine
having a molecular weight from 103 to 106 are more
desirable. Among the amine polymers shown hereinbelow,
polyaminesulfon (hereinafter called PAS) and polyamine
(hereinafter called PA) are most effective to promote
the precipitation of Cr. This is attributable to the
adsorption effect by the amine group and the combination
of the sulfo group with the metal ions or with the metal.
Basically, the cation polymer used in the
present invention is selected from a homopolymer or
copolymer having in its main chain a salt of quaternary
amine (ammonium salt) shown below.
R `+
R4 - N - R2 -~ X
~ R3
where R1, R2, R3 and R4 are each an alkyl group having
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-
1 to 4 carbon atoms.
Some typical examples of the c`ation polymers
used in the present invention will be illustrated below.
(1) Polymers obtained from diallylamine:
CH
CH CH - CH2 ~n )PA
\ /
N j X
R1 R2
S
CH I2 O// ~O ~n ~ PAS
\ /
N j X
R1 R2
wherein Rl and R2 are each a lower alkyl group (CH3,
C2H5, C3H7, and C4Hg), and X stands for Cl , HSO4 ,
H2pO4 , R-SO3 (R is an alkyl having 1 to 4 carbon atoms)
and anion of NO3 .
(2) Polymers obtained from vinylbenzyl.
CH2 - CH ~n
PB
N X
/ I \
R1 R2 R3
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,
(3) Allylaminepolymers
CH2 - CIH ~n ) PAR
N --.-. X
/ I \
R1 2 3
wherein R1, R2 and R3 are each an alkyl group (CH3, C2H5,
C3H7, or C4H9) and X represents anions of Cl , HS04 ,
H2P04 , R-S03 (R is an alkyl group having 1 to 4 carbon
atoms) or N03 .
In addition to the above quaternary amine
polymers, polymers of primary, secondary and tertiary
amines may also be used for promoting the precipitation
of Cr although they are less effective.
Regarding the amount of the prlmer coating,
10 to 50 g/m2 is enough for assuring the desired corrosion
resistance, and unavoidable impurities such as`Pb, Sn,
Ag, In, Bi, Cu, Sb, As, Al, Ti, Na, P and S may be present
in a minor amount in the primer coating without degrading
the desired properties of the present coated products.
For improving the spot weldability of the coated
steel sheets according to the present invention, the
ion-group metal, such as Fe, Ni and Co, is co-precipitated
in the primer coating in an amount of 1 to 10 ~ by weight.
The Zn-Cr electro-plated coating containing no iron-group
metal show inferior spot weldability as compared with
the conventional Zn-Ni and Zn-Fe alloy coatings. Although
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not theoretically clarified, the reason for the inferior
spot weldability may be attributed to the following facts
that as compared with the conventional Zn-Ni and Zn-Fe
coatings the Zn-Cr coating containing no iron-group metal
has a lower electric resistance and is more easily heated
and fused by the current passage and further the coating
itself is softer and is easily deformed by the pressure
exerted thereon by a welding tip so that the current
can hardly be concentrated on the welding spot.
When the Zn-Cr coating contains the iron-group
metal, such as Fe, Ni and Co, the electric resistance
is enhanced and the coating itself is hardened so that
the spot weldability is definitely improved. With the
iron-group metal content in an amount less than 1 % by
weight is not effective for the purpose, while in excess
of 10 % by weight, the coating is influenced largely
by the nature of the iron-group metal so that corrosion
resistance is, in some cases, deteriorated.
When the iron-group metal is present in the
electro-plating bath, it produces favorable effect that
the ions of this metal adsorbs on the fine particles
of oxide, if present, to facilitate the precipitation
of the oxide. However if the total amount of the
iron-group metal in the coating is excessively increased,
the workability of the coating is deteriorated despite
the favorable effect of the co-precipitation of the cation
polymer. Therefore, it is desirable to maintain the
total content of Cr and the iron-group metal in the
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coating not more than 30 % by weight. As the iron-group
metal, Ni is most desirable because it can further improve
the corrosion resistance of the coating.
The fine particles of oxide contained in the
coating contribute to further stabilize the corrosion
products of Zn-Cr by intrusion thereinto and strong
combination therewith through oxygen bonding, and the
fine particles of oxide act as a barrier against corrosive
factors. In this way the corrosion resistance of the
coated steel sheet, particularly the corrosion resistance
at and around the worked portions, is improved.
For the above purpose, it is desirable that
the fine particles of oxide are contained in the coating
in an amount from 0.1 to 10 % by weight. Less than 0.1 %,
no substantial improvement of corrosion resistance is
obtained and more than 10 %, workability is deteriorated.
From aspects of both corrosion resistance and workability,
0.1 to 5 % by weight of the fine particles of oxide is
more preferable.
As the fine particles of oxide usable in the
present invention, many metal oxides and semi-metal oxides
may be used, but oxides of Si, Al, Zr, Cr, Mo and W are
most preferable. These oxides can be used in single
or in combination. The particle size of these oxides
is preferably not larger than 1 ~m in average. The
particle size larger than 1 ~m is hard to co-precipitate
in the coating.
The primer coating according to the present
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_ 1 337555
invention may be of homogeneous or heterogeneous
structure. Thus within the scope of the coating
composition as defined hereinabove, one or more specific
components may be dispersed or condensed in a layer form,
or the concentration of specific component or components
may vary in gradient across the coating thickness.
The coated steel sheets having the primer
coating described hereinabove may further be coated with
a Zn or Zn-alloy coating such as Zn-Fe and Zn-Ni coatings
in a small amount.
~ urther, instead of the Zn or Zn-alloy coating,
the primer coating may be subjected to a chromate
treatment and then further coated with an organic coating.
When the Zn or Zn-alloy coating is applied
o~ the primer coating, 1 to 5 g/m2 of such coating is
preferred to improve the corrosion resistance after paint
coating under a wet environment as under a salt spray
test condition.
The chromate film applied on the primer coating
is effective to enhance the adhesion with the organic
coating.
As the Zn-Cr coating is highly reactive with
an acidic treating solution containing Cr and/or Cr3 ,
any of the conventional chromate treatments, such as
the spray type chromate treatment, the reaction type
chromate treatment, and the electrolytic type chromate
treatment may be applied.
For the spray type chromate treatment, and
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1 337555
the reaction type chromate treatment, inorganic colloids,
acids such as phosphoric acid, fluoride, water-soluble
or emulsion type organic resin may be added to the
treating solution in addition to Cr6 and Cr3 as
conventionally done.
For example, as the solution containing the
phosphoric acid and the fluoride, the solution may be
composed of 30 g/l of chromic acid, 10 g/l of phosphoric
acid, 4 g/l of potassium titanate fluoride, and 0.5 g/l
sodium fluoride.- As the treating solution containing
the silica, the solution may be composed of 50 g/l of
chromic acid (including 40 ~ of trivalent chromium) and
100 g/l of SiO2.
As the inorganic collolds, preferred examples
are colloids of SiO2, A12O3/ TiO2 and ZrO; colloids
containing one or more of oxyacids such as molybdic acid,
tungstic acid, vanadic acid, and their salts; phosphoric
acids such as phosphoric acid and polyphosphoric acid
which form hard-to-dissolve salts by reaction with Zn
of the coating; and silicofluorides titanium, and
phosphates which form hard-to-dissolve salts by reaction
such as hydrolysis.
These colloids are found to be effective to
fix a small amount of hexavalent chromium in the chromate
film and also phosphoric acids and fluorides mentioned
above in particular are found to be effective to promote
the reaction between the primer coating and the chromate.
The amount of these inorganic colloids to be
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added may vary depending on their natures; in the case
of phosphoric acids, 1 to 200 g/l is desirable, and in
the case of SiO2, 1 to 800 g/l is desirable, for example.
In some cases, organic resins, such as acrylic
resin, which are consistently mixable with the chromate
may be added.
As the electrolytic chromate treatment, any
conventional treatment can be applied, such as one in
which sulfuric acid, phosphoric acid, and halogen ions
are added in addition of chromic acid, or inorganic
colloids such as SiO2 and A12O3 are added, or cations
such as Co and Mg are added. Normally the electrolysis
is performed by the cathodic electrolysis, but the anodic
electrolysis and alternate current electrolysis may be
additionally used.
The amount of chromate film formed on the primer
coating is preferably from 10 to 150 mg/m2 in the term
of total chromium. With a chromate film less than
10 mg/m2, the adhesion of the organic film is not
satisfactory, while a chromate film more than 150 mg/m2,
the weldability and press formability deteriorate.
Therefore, from the practical point, 20 to 100 mg/m2
is more desirable.
In order to avoid the contamination of the
chemical conversion treating solution with chromium
dissolving from the chromate film, and the complicated
handling of the waste liquid, it is advantageous to form
a chromate film which contains not more than 5 ~ of water-
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1 337555
soluble component. For forming this hard-to-dissolve
chromate film, the electrolytic chromate treatment is
better suited.
On the chromate film, an organic coating of
0.3 to 3 ~ in thick may be applied for improving the
corrosion resistance. With an organic coating less than
0.3 ~ the desired improvement of corrosion resistance
cannot be assured, and with an organic coating
exceeding 3 ~, weldability and press formability may
sometimes deteriorate. A more preferable range is from
0.5 to 2 ~.
The organic coating may be either a solvent
type or a water soluble type, and epoxy and acrylic
resins, polyester, urethane and acrylic olefins etc.,
and their copolymer derivatives may be used, for example.
For heat-curing type organic coatings containing
rust preventive pigments such as SiO2 and BaCrO4, various
additives such as curing agent and lubricant for further
improving the press formability may be added.
One preferred embodiment of the organic coating
usable in the present invention is illustrated below.
Main resin: bisphenol type epoxy resin (average
molecular weight: 300 to 100,000),
30 ~ or more by weight in the
coating.
Curing Agent: block polyisocyanate compound:
1/10 to 20/10 by weight ratio to
the main resin.
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1 337555
`_
Rust preventive: dry silica (average pFimary
pigment particle size 1 to 100 ~m);
5 to 50 % by weight to the coating.
Lubricant: polyethylene wax; 0.1 to 10 % by
weight to the coating.
Solvent: ketone
The organic coating may be applied by any
conventional methods, such as roll coating, spray coating,
and curtain flow coating.
According to the present invention, the coated
steel sheet may be coated only one side, and the other
side may be uncoated or coated with the Zn-Cr coating
alone depending on the final applications the coated
steel sheet is intended for.
For producing the coated steel sheet according
to the present invention, the electo-plating may be
performed in a plating bath containing Zn ions,
Cr ions, and 0.01 to 20 g/l of water soluble cation
polymer, such as copolymers of tertiary amines as
mentioned hereinbefore, having a pH value ranging from
0.5 to 3 at a bath temperature ranging from 40 to 70C
with a current density of 20 A/dm or higher. '
As the case requires, the iron-group metals,
and fine particle oxides, such as SiO2, TiO2 and A12O3
are added to the bath. Further the addition of salts
of Na , K , and NH4 ions are advantageous for improving
the electric conductivity of the bath.
The Zn coating or Zn-alloy coating to be formed
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on the primer coating may be done by a conventional
plating method. In this case, however, it is most
desirable to completely remove the dragout containing
Cr ions and cation polymer taken out from the primer
coating bath with water prior to the coating.
As for the substrate for the coated steel sheet
according to the present invention, a dull-finished rolled
soft steel sheet is normally used. However, a
bright-finished rolled steel sheet may be used, and the
10 steel composition may be a high tensile steel containing
larger amounts of Mn, S and P or may be a corrosion
resistant steel containing larger amounts of Cr, Cu,
Ni and P.
Brlef Explanation of the Drawings:
Fig. 1 is a drawing showing the shape of welding
tip used for evaluating the spot-weldability.
Fig. 2 is to show the correlation between the
surface brightness and the Cr content and the ratio of
Cr6 /Cr3 and in a typical plating bath used in the
present invention, showing that the ratio of Cr6 /Cr3
not more than 0.1 is essential.
Detailed Description of the Preferred Embodiments:
The present invention will be described in
more detail in connection with the embodiments.
, r. - 24 -
..
_ - 1 337555
Example 1
A cold rolled steel sheet with a thickness
of 0.8 mm was subjected to alkali degreasing, acid
pickling with the use of 5 % sulfuric acid solution and
washing in succession, and then the resultant steel sheet
was electro-plated in an acidic bath of sulfuric acid
at pH 2 at a bath temperature of 60C, with a flow rate
of the solution of 90 m/min by pump stirring, and the
distance between the electrodes of 10 mm. The plating
10 bath was composed of 70 g/l of zn2 ion, 1 - 30 g/l of
Cr3 ion, 0.01 - 20 g/l of cationic polymer (polyamine
polymer (PA) with molecular weight of lO,OOO or
polyamine-sulfone polymer (PAS) with molecular weight
of 120,000) and 16 g/l of Na . The contents of Cr and
cationic polymer were controlled by their addition amounts
and the electric current density. The plated amount
was 20 g/m . In certain instances, further, Zn or
Zn-alloy coating was applied in an amount of 3 g/m2 as
a topcoat by means of a known method.
The coating compositions and evaluation of
corrosion resistance and workability of the electro-plated
steel sheets produced in this way are shown in Table 1.
By the way, Zn and Cr in the coating were analysed by
means of the atomic absorption method and, as for the
cationic polymer, C was analyzed by the combustion method
and the amount of cationic polymer was recalculated
therefrom. The methods for the evaluation of corrosion
resistance and workability are as follows:
, . ~ . .
- 25 -
1 337555
-
(1) Corrosion resistance of non paint-coated samples:
(a) Salt spray test (SST) (in accordance with
JIS Z2371): Evaluated by the area of red rust formed
after 672 hours.
less than 1 %: ~
1 9~ - 10 %: O
10 ~ - 30 ~
more than 30 %: X
(b) Cyclic corrosion test (CCT):
~ Salt spray (JIS Z2371) for 4 hours
Drying (60C) for 4 hours
- Moisture treatment (50C, RH ~5 %)
for 4 hours
By taking the above three steps as one cycle, the results
were evaluated by the loss of sheet thickness after
30 cycles.
less than 0.1 mm: ~
0.1 mm - 0.2 mm: O
0.2 mm - O.3 mm: f~
more than 0.3 mm: X
(2) Corrosion resistance after paint coating:
The Zn-Cr coated samples were subjected
successively to immersion type phosphoric acid treatment,
cathodic electric coating of Zn with a thickness of 20 ~m,
intermediate coating, water polishing and topcoat paint
coating to obtain a total thickness of coatings of 100 ~m.
The test pieces were given a crosscut reaching to the
base metal and subjected to the 1,000 hour SST and the
- 26 -
~,~
1 337555
-
60 cycle CCT as above mentioned, and the results were
evaluated by means of the swollen width of the crosscut
part.
less than 1 mm: ~
1 mm - 3 mm: O
3 mm - 5 mm:
more than 5 mm: X
(3) Workability
After cylindrical press forming with a size
of 50 ~ x 25 H, tape peeling tests were done on the worked
surfaces, and the results were evaluated by the weight
losses.
less than 2 mg: ~
2 mg - 5 mg: O
5 mg - 8 mg:
more than 8 mg: X
All the examples of the present invention were
obviously excellent both in corrosion resistance and
workability as compared with the comparison examples.
t C - 27 -
T a b I e
Corrosion Resistance Corrosion Resistance
No Prioer Coating Conposition(~ei~ht X) Kind of Upper before Paint Coating after Paint Coatin8 Workability
Layer Coating
Zn Cr Cation Polyoer Others S S T C C T S S T C C T
I balance S PA 0 005 0
2 ~1 10 O 0 005 0
3 ~ 20 ~ ~ ~
4 ~I 30 1~ 3 ~ ~ O ~ O
S 1I S PAS 0 005 O O O O O
6 11 10 11 1 ~ ~
Exaaple
7 1l 20 11 a ~ ~ ~
1 8 1~ 30 11 S ~ ~ O ~ O
g It 7 ~1 o 01 ~ O O ~ O
1 10 1~ 7 1~ 0 1
~ 7 ~ O
12 ~ lS Il 0 05 ~ ~ O ~ O
13 ~ lS ~ 0 5 ~ ~ O
14 ~ 15 ~ 5 ~ ~ O
~ 7 ~ 0 01 2n 0 0 0 0 O ~J~
16 1~ 7 ~ 0 1 Zn-llSNi O O 0 0 0 ~J~
17 ~ 7 ~ 1 Zn-80%Fe O O 0 0 o ~J'
18 ~ IS ~ 0 05 Zn O O O O O
13 ~ IS ~ 0 5 Zn-llXNi O O O O O
~ lS ~ S Zn-80XFe O O 0 0 0
T a b 1 e 1 (Cont'd)
Corrosion Resistance Corros;on Resistancc
No. Priner Coatin8 Co~position(~eight X) Kind of Uppcr before Paint Coating after Paint Coating Workability
Layer Coating
Zn Cr Cation Poly~er Others S S T C C T S S T C C T
21 100 - - - x x x x O
22 bal~nce - - - Ni.12 x x
23 J~ - _ _ Fe,15 Zn-80SFe x x
Con p8 rison
24 ~ 5 - - O O O O x
11 I PAS 0.03 ~ x ~ x O
26 ~ 40 PAS 0.5 0 0 0 0 x
27 ~ 10 PAS 0.001 0 O O O x
I
~n
Example 2 l 337555
Except the addition of 10 - 150 g/l of iron
group metal ion to the plating bath composition, plating
was done under the same conditions as in Example 1.
The coating compositions and evaluation of
corrosion resistance, spot weldability and workability
of the electro-plated steel sheets produced in this way
are shown in Table 2. Zn, Cr, Fe, Co and Ni in the
coating were analyzed by means of the atomic absorption
method and, for the cationic polymer, the amount of carbon
was analyzed by the combustion method and the amount
of cationic polymer was recalculated therefrom. The
methods for evaluating corrosion resistance and
workability were the same as in Example 1. Spot
weldability was evaluated by the following method.
Spot Weldability:
The welding conditions were as below.
Electric current: 8 kA
Number of cycle: 10 cycles
Pressure applied: 200 kg
Shape of welding tip: as shown in Fig. 1 (where
A is 12 mm~, B is 6 mm~
and ~ is 30)
After continuous shock of 5,000 times, the nugget diameter
was measured and the results were evaluated as follows.
not less than 3.6 mm: O
less than 3.6 mm: X
- 30 -
., < . ~
T a b I e 2
Priner CoatinR Co-position(veight X) RCo rosion Corrosion
No. Kind of Uppcr be ore Reslstance after Spot
Iron- Layer Coating Coa.ing Paint Coatin8 Workab;ljty W~kability
Zn Cr ~roup Cation Polyner
~ietal S S TS S T C C T .
2- 1balance 5 Ni:3 PA 0.01 O O O O
2- 21l 5 Ni:3 ~ O.l O O O O
2- 3~J 5 Fe:3 ~ O.Ol O O O O O
2- 4 ~ 5 Co:3 ~ O.Ol O O O O O
2- 511 7 , û PAS O.01 ~ O O O
2- 61~ 15 ~ O.Ol O O ~ O
Exa-ple 2 2- 7 Jl 25 Ni:3 1l 3 o O O O O
1 2- 811 29 Ni:l ~ S ~ O O O O
w 2- 9/~ 5 Ni:l ~ O.OOS O O O O O
1 2-101~ 5 Ni:2 ~ 0.05 O O O O O
2-111~ 5 Ni:S J~ 0.5 O O O O
2-1211 lû Ni:l ~ O.Ol o O O O
2-131l 10 Ni:S 1~ O.l ~ o ~ O
2-141l 10 Ni:10 1~ I ~ o ~ O O
2-15~1 5 Ni:l ~ O.O05 Zn O O O O O
2-161~ 5 Ni:2 ~ O.OS Zn-llXNi ~ O O O O
2-171~ 5 Ni:S ~ 0.5 Zn-8û%Fe O O O O O
2-181~ 10 Ni:l ~ O.OI Zn O O O O O ~J~
2-191~ lO Ni:S ~ 0.1 Zn-llXNi O O ~ O O ~1
2-201~ 10 Ni:10 ~J 1 Zn-80XFe O O O O
T a b I e 2 (Cont'd)
Cn ~ ~oS ion
Prioer COatinK Co~posltiont~ei8ht X) Re- -tance CorrosionNo. Kind of Upper a nt Paint Coatin~ Spot Workability
Iron- Layer Coatin~ Coa.ing Workability
Zn Cr ~roup Cation Poly-cr
~etal S S T S S T C C T
2-21 100 - ~ ~ ~ x x x x O
2-22 balance - Ni:12 - - x ~ ~ O
2-23 1~ - Nl:lS - - Zn-80SFe x ~ ~ O
Coaparison
2 2-24 ~ S - - - o O O x x
2-25 IJ INl:S PAS O.S ~ ~ x O O
2-26 Jl 10Nl:0.1 ~ I O O O x O
1 2-27 ~ 35Ni:S ~ 3 O O O O x
2-28 ~1 10Ni:S ~ 0.001 0 O O O x
~n
~n
1 331555
Example 3 (Zn plating + chromate treatment)
Except the addition of 0 - 30 g/l of iron-group
metal to the plating bath composition, plating was done
under the same conditions as in Example 1. To the plated
steel sheets, chromate treatment and organic coating
were further applied as shown below.
(1) Chromate treatment:
(a) Electrolysis-type chromate treatment:
By using a treating solution containing 3.0 g/l
10 Of chromic acid and 0.2 g/l of sulfuric acid, the plated
steel sheet in the above was subjected to cathodic
electrolysis with a current density of 10 A/dm at a
bath temperature of 40C, and the resultant product was
washed and dried. The amount of chromate to be formed
was controlled by means of the amount of coulomb.
(b) Coating-type chromate treatment:
The coated steel sheets were immersed in a
treating solution comprising 50 g/l of chromic acid
(containing Cr3 40 %), 100 g/l of colloidal SiO2 at
a bath temperature of 40C. The resultant coated steel
sheets were dried for 1 min at 100C after air-wipe
treatment. The amount of chromate to be formed was
controlled by means of the dilution ratio of the treating
solution and the pressure at the air-wipe treatment.
(c) Reaction-type chromate treatment:
A treating solution containing 50 g/l of chromic
acid, 10 g/l of phosphoric acid, 0.5 g/l of NaF and 4 g/l
of K2TiF6 was sprayed on coated steel sheets at a bath
. - 33 -
1 337555
temperature of 60C, and the resultant coated steel sheets
were dried at 60C after washing. The amount of chromate
was controlled by means of the dilution ratio of the
treating solution and the duration of its spraying.
(2) Organic coating:
The coated steel sheets obtained by the above
mentioned chromate treatment were further subjected to
organic coating under the following conditions.
While the kind of resin as the main constituent
10 is shown in Table 3, such agents as a rust preventing
pigment, for instance SiO2, curing agent, catalyst,
lubricant and reformer against wetting were also added
in the paint. The coated steel sheets to which chromate
treatment was applied were further coated with the paint
by means of a roll-coater, baked and dried. The baking
condition differed in accordance with the type of resin
used, but final sheet temperature was selected in the
range of 100 - 200C.
The construction of the thus obtained coated
20 steel sheets having organic coating and the result of
evaluation of their corrosion resistance, workability
and weldability are shown in Table 3. The methods for
evaluation are as below.
(a) Corrosion resistance of non worked portions:
The evaluation was done by the cycle corrosion
test (CCT) as specified in Example 1.
(b) Corrosion resistance of worked portions:
After cylindrical press forming with a size
- 34 -
1 337555
-
of 50 mm~ x 25 mmH, the salt spray test (JIS ZZ371) was
done for 2,000 hours, and the evaluation was done with
respect to the red-rust-suffering area in the worked
portions.
less than 1 %: ~
1 % - 5 %: O
5 % - 10 %:
more than 10 %: X
(c) Press workability:
Evaluation was done by the workability
evaluation method as specified in Example 1.
(d) Spot weldability:
Welding was done under the conditions shown
in Example 2, and the evaluation was done by means of
the number of times in continuous shock.
number of times more than 5,000: ~
" 4,000 - 5,000: O
" 3,000 - 4,000:
" less than 3,000: X
The results of these tests are shown in Table 3.
As for comparison examples, the Cr content in the primer
coating of No. 3-17 is too low, the iron group metal
content (Ni) in the primer coating of No. 3-20 is too
high, the organic coating film in No. 3-24 is too thin,
and the primer coating in No. 3-26 contains no Cr, so
that the corrosion resistance of these samples is not
good. On the other hand, the Cr content in the primer
coating of No. 3-18 is too high, No. 3-19 contains no
- 35
._ . .
1 337555
cationic polymer in the primer coating, No. 3-21 contains
an excessive amount of Cr and iron group metal (Ni) in
total in the primer coating and No. 3-22 has too small
amount of chromate film, and consequently their press
workability is inferior and the corrosion resistance
is also low. Further, since No. 3-23 has too much amount
of chromate film, and No. 3-25 has too much organic
coating film, their spot weldability is inferior.
In contrast to these comparison examples, as
obvious from Table 3 all examples according to the present
invention are excellent in all the points of corrosion
resistance, workability and weldability.
. - 36 -
~ , .. .
T a b I e 3
Priner Coatinx Conposition (other than Zn) Chro~ate Fila Or~anic Fil~
Corrosion Corrosion
No Iron-group Metal Cation Polyeer A~ount of Cr Resistance Resistance Press Spot
Cr TypeDeposition Yain PolynerThickness before after Forrability Workability
(~ei8ht X) kind (~eight S) kind (~eight X) (rg/o2) Co-ponent (~) Workin~ Working
3- 1 5 - - PAS 0 05 Electrolysis 20 Epoxy 1 0 0 0 0 0
3- 2 10 - - PA 0 5 IJ 40 Acrylic 1 2 69 0 0 0
3- 3 20 - - PAS 2 Reaction 50 Urethane 1 2 0 63 0 0
3_ 430 ~ 5 Spray 40 Acrylic-Olefinic 1 2 0 0 0 0
3- 5 5 Ni 2 1~ 0 05 Electrolysis 40 Epoxy 1 0 ~ 0 0 0
3- 6 5 Fe 2 PA 0 1 Reaction 80 Acrylic 0 7 0 0 69 0
Exaxple 3 3- 7 5 Co 2 PAS I Spray 30 i)rethane 0 7 0 O O
3- 8 7 Ni 3 ~1 0 05 Electrolysis 40 Epoxy 2 0 0 0 0 0
Ni 2
3 9 7 Fe I ~ 0 005 ~ 10 ~ I 0 0 0
1 3-10 7 cNO 21 PA 0 2 1l 150 ~ 1 0 0 0 0 0
~J 3-11 7 Ni I PAS 0 1 11 40 ~1 3 0 0 0 O O
1 3-12 10 Ni 3 ~ 0 01 ~ 50 1l 0 3 0 O O O
3-13 10 Fe 5 ~J 2 Reaction 100 Acrylic-Olefinic 0 7 0 0 0 0
3-14 10 Co S PA 0 5 Spray 20 Epoxy 0 7 0 0 O O
3-15 20 Ni 2 PAS I Electrolysis 40 1l 1 5 0 6~ O O
3-16 20 Ni 10 ~ 5 11 50 u 0 5 0 63 0 0
3-17 0 1 - - ~ 0 05 ~ 60 ~ 1 0 x x O O
3-18 35 - - ~ 3 ~ 20 ~ I O ~ x O
3-19 5 ~ 40 ~ 1 0 ~ x O
Co-parison 3-20 10 Ni 10 PA$ 0 5 IJ 40 ~ 1 0 ~ ¢9 O _ ,
3 3-21 30 Ni 15 1l I 11 40 11 1 0 69 ~ x O <_r~
3-22 5 - - ~ 0 05 ~ 5 ~ I o O ~ x O ~_rJ
3-23 5 - - ~ 0 05 ~ 200 ~ I 0 69 0 ~ x
3-24 5 Ni 2 ~ 0 05 ~ 40 1~ 0 1 x x ~9 ~ ~ rl
3-25 5 Ni 2 ~ 0 05 ~ 40 ~ 3 5 69 ¢9 x x ~Jr
3-26 - Ni 11 - - 11 40 ~ 1 0 ~ x O O
1 33~555
Example 4
Except the addition of 0 - 50 g/l of iron group
metal ion and 10 - 100 g/l of fine oxide particles (SiO2,
A12O3, ZrO2 and TiO2 having a mean particle size of 0.02
- 0.05 ~, and Cr2O3 and WO3 having a mean particle size
of 0.1 - 0.5 ~ in the plating bath composition, plating
was dione under the same conditions as in Example 1.
The compositions of primer coatings and results
of evaluation are as shown in Table 4. By the way, the
10 methods for determining metal constituents as well as
the cationic polymers used are same as in Examples 1
and 2. The methods for evaluating corrosion resistance
and workability are as below.
(1) Corrosion resistance of nonpaint-coated sample:
Salt spray test (exposed at Chiba district
by spraying 5 % saline water once a week) was done, and
the results were evaluated by the area where red rust
formed after one year exposure.
less than 1 %: ~
1 % - 10 %: O
10 % - 30 %:
more than 30 %: X
(2) Corrosion resistance after paint coating:
The result was evaluated according to the method
as specified in Example 1.
(3) Spot weldability:
The welding was done under the conditions as
shown in Example 2, and the results were evaluated
- 38 -
.. ..
1 337555
according to the following method, i.e., to measure the
diameter of nugget after continuous shock of 3,000 times.
more than 4 mm: ~
3 mm - 4 mm: O
less than 3 mm: X
(4) Workability:
Evaluated according to the method as specified
in Example 1.
The results are shown in Table 4. All the
10 examples according to the present invention are excellent
in corrosion resistance and weldability as compared with
the comparison examples. The examples containing iron
group metal are particularly excellent in their spot
weldability, and the examples having topcoat coating
are also particularly excellent in the corrosion
resistance after paint coating.
- - 39 -
T a b I e 4
Corroslon
Prieer Coating Couposition (~eight X) Resistance Corrosion
Kind of Upper before Resistance after Spot
No. PaintPaint Coating Workahility
Iron- Fine Layer Coating Coatlng ~orkability
ZnCr ~roup Particle Catlon PolY-er
~etal Oxide Exposure S S T Exposure
4- 1balance 5 - - SiO, 2 PA 0.1 0 O O O
4- 2 ~ 10 - - TiO~ 2 ~ 0.5 0 0 0
4- 3 ~1 20 - - ZrO2 2 ~ I O O O O O
4- 4 1l 29 - - Al10, 2 PAS 5 0 O O O O
,j_ 5 ~ 5 _ _ A~OO I ~ 0.5 ~ O O O
4- 6 ~ 10 - - SiO2 0.1 ~ 0.5 ~ O O O
I Exaaple 4
4- 7 ~ 15 - - ~ 5 ~ O ~ O
4- 8 ~ 5 - - ~ 10 ~ 0.01 0 0 0 0 0
4- 9 ~ 15 - - ~ 0.1 ~ 2 2n 0 0 0 O
4-10 ~1 5 - - ~ 0.05 Zn-12XNi O O O O O
4-11 ~ 10 - - ~ 3 ~ 0.1 2n-85XFe O O O O O
4-12 11 5 Nl10 SiO2 3 PA 0.1 0 0 0 0 0
Ni 5
4-13 ~ 7 Fe I ~ 2 3 ~ 0 3
4-14 ~ 10 CoSi ZrO. 3 1~ 0.5 ~ O O
4-15 /~ 20 Co 5 ~0, 3 P~S 3 ~ O
4-16 ~ N 3 TiO2 1 ~ ~ O O O
4-17 ~ 5 ~1 3 S~02 0.1 Jl 0.05 ~ ~ ~ ~
T a b 1 e 4 (Cont'd)
Corrosion
Priaer Coating Conposition t~eight X) Resistance Corroslon
No Kind of UpperbeforeResistance after Spot
PaintPaint Coating Workability
Iron- Fine Layer Coatin8 Coating ~orkability
ZnCr group Particle Cation Poly-er
~ietal Oxide Exposure S S T Exposure
4-18 balance 7Ni 2 SiO, 5 PAS 0 5 ~ o ~ ~ O
4-lg 1~ 10 ~ 4 1l10 ~ 0 5 0 0 0 ~ O
Exaaple 4 ri-20 ~ 25J~ 5 Jl 3 IJ 5 Zn O O O O O
4-21 ~J 7 JJ2 ~ 3 ~ 0 3 2n-12SNi O O O
4-22 ~ 10 IJ3 JJ 3 JJ I JJ O O
4-23 JJ 5 ~10 ~ 3 ~ 0 01 ~ ~ O
4-24 JJ 7 JJ5 JJ 3 JJ 0 1 J~ ~ O O
4-25 JJ 10 Jl3 ~ 3 ~J 2 2n-85~i'e ~ O O O O
4-26 100 - - - - - - - x x x x O
4-27 balance - Ni 15 - - - - x ~ ~ O O
4-2li JJ10 - - - - PAS I ~ O ~ O
Couparison 4-29 JJ0 1 Ni 3 SiO2 3 Jl 0 01 x x x O O
4 4-30 Jl 35 - - Ai20, 2 ~ 5 0 O O O
4-31 ~ 5 Ni15 ZrO2 3 J~ 0 1 ~ O O O x
4-32 Jl 10 113 TiO20 01 J/ 1 ~ O ~ O O
4-33 11 10 JJ3 Cr20, 15 1l 1 0 O O O x
4-34 11 10 - - SiO2 2 - ~ ~ O O x
_ 1 337555
Example 5
Plating was done under the same conditions
as in Example 4, and the resultant coated steel sheets
were subjected to chromate treatment and organic film
coating treatment in succession under the same conditions
as in Example 3.
The construction of thus produced composite
coated steel sheets having organic film coating and their
corrosion resistance, workability and weldability were
evaluated as shown in Table 5. The methods for evaluation
are as below.
(a) Corrosion resistance of flat sheet:
-~Immersion in saline water (40C, 5 % NaCl)
for 10 min.
Drying (60C) for 10 min.
- Moisture treatment (50C, RH 95 %)
for 10 min.
By taking the above three steps as one cycle, the results
were evaluated by the loss of sheet thickness after
3,000 cycles.
less than 0.1 mm: ~
0.1 mm - 0.2 mm: O
O.2 mm - O.3 mm:
more than 0.3 mm: X
(b) Corrosion resistance of the worked portions:
After cylindrical press forming with a size
of 50 mm0 x 25 mmH, the salt spray test (JIS Z2371) was
done for 3,000 hours, and the evaluation was done with
- ; - 42 -
. .~ ,, ~,
. ~ .
1 337555
respect to the red-rust-suffering area at the worked
portions.
less than 1 %: ~
1 % - 5 %: O
5 % - 10 %:
more than 10 %: X
(c) Press workability:
Evaluation was done by the method as specified
in Example 1.
(d) Spot weldability:
Welding and evaluation were done similarly
as in Example 3.
The results of these tests are shown in Table 5.
As for comparison examples, No. 5-20 is low in Cr content
in the primer coating, No. 5-22 is too high in the content
of iron group metal (Ni) in the primer coating, No. 5-23
and 5-24 contain no fine oxide particles in the primer
coating, No. 5-28 is too thin in its organic coating
film and No. 5-30 contains no Cr in the primer coating,
so that they are all inferior in corrosion resistance.
On the other hand, No. 5-21 contains too much Cr in the
primer coating, No. 5-25 contains no cationic polymer
in the primer coating, and No. 5-26 is too thin in its
chromate film, and therefore their press workability,
and accordingly, corrosion resistance at the worked
portions are inferior. Further, since No. 5-27 has too
much chromate film and No. 5-29 is too thick in its
organic film, their press workability and spot weldability
- 43 -
,
1 337555
are not good.
In contrast to these comparison examples, all
examples No. 5-1 to 5-19 according to the present
invention are superior in corrosion resistance,
workability and weldability.
~` - 44 -
._,, .~
Tab le 5
Priner Coatin8 Co~position Chrooate Filn Organic Fil-l
Corrosion Corrosion
No.Iron-~roup Metal Fine Particle Oxide Cation Poly~er A~ount of Cr Resistance Resistance Press SpotZn Cr Type Deposition Main Polyoer Thicknessof Planar of ~lorked Fornability Workability
(~1eight X) kind (l~ei8ht X) kind (~ei8ht X) kind (~eight X) (~2) Cc~p~nent (JL) Portion Portlon
5- 1balance 5 ~ - ilO, 2 PAS O.ûS Electrolysls 40 Urethane 3.0 0 0 0 0
5- 2JJ 7 - - SiO2 1O JJ 0.1 J~ 60 Epoxy 1.0 0 O O O
5- 3JJ 10 - -- Jl 5 PA 0.5 IJ 80 J/ 0.3 0 0 0 O
5- 4IJ 20 - - IJ 0.1 PAS 1 Jl 20 11 0.5 0 0 0 0
5- 5 ~ 30 ~ ~ TiO2 1 ~ 5 Spray 10 Jl 1.0 0 0 0 0
5- 6IJ 5 Ni 2 SiO2 2 ~ 0.01Electrolysis 60 ~J 3.0 0 0 0 0
5- 7ll 5 Fe 2 ll 2 ll 0.5 ll 80 ll 1.0 0 0 0 0
5- 8J/ 5 Co 2 J/ 2 PA I Spray 150 ll 1.0 0 O O O
~n
5- g11 7 Fe 21 ~0, 3 PAS 0.05Electrolysis 20 Acrylic-Olefinic 1.0 0 0 0 0
Exaople S
5_10 ll 7Co 2 Cr20. 3 ll 0.5 ~ 100 Urethane 0.3 0 0 0
5-11Jl 7 Ni 3 SiO~ 10 Jl 0.01 ~ 50 Epoxy 1.0 0 ¢9 0
5-12Jl 7 Ni 5 JJ 5 JJ I ~ 50 J~ O. S O ~D O O
5-13Jl 7 Ni 3 JJ 0.1 PA 0.1 Jl 50 JJ 1.0 0 6) 0 0 ~J
5-14JJ 10 Ni 2 TiO2 2 PAS 0.01 JJ 150 IJ 1.0 E9 69 0 0
5-15JJ 10 Fe 5 ~120~ 2 IJ 1 Reaction 40 Acrylic 0.7 0 O O
5-16JJ 10 Co 5 ZrO2 2 JJ 2 Electrolysis 50 Polyester 1.5 0 0 0
5-17IJ 10 Ni S SAI200. 2 PA 0.1 Jl 10 Epoxy 1.0 0 0 0 0
5-18JJ 20 Ni 2 SiO2 I PAS I 11 50 11 1.0 0 g9 0 0
5-1911 20 Ni 10 1l 2 11 5 Spray 40 ~ 1.0 0 19 0 0
-
Ta b I e 5 (Cont d)
Primer Coating Composition Chromate Film Organic Film
Corrosion Corrosion
No.Iron-group tletal Fine Particle Oxidc Cation Polymer Amount of Cr Reslstance Resistance Press Spot
Zn Cr tYpe 0eposition~lain Pol~er thickness of Planar of ~orked FormabilitY Workability
(~/eight X) kind t~eight X) kind (l1eight X) kind (l~eight %) Wl~) Cooponent (~) Portion Portion
5-20balance 0.1 - _ S102 2 PA 0.001 Electrolysls 50 EpoxY 1.0 x x O O
5-21IJ 35 - - Ah0, 2 JJ 5 ~1 50 JJ 1. 0 O ~ x O
5-22~1 5 Nl15 2rO~ 5 ~ 0.5 ~ 50 ~ 1.0 ~ ~ x
5-23~) 10 - ~ 50 1~ 1 . 0 ~ ~ ~ O
Comparlson 5-24tl 7 Ni 5 - - ~ 0.5 J~ 50 JJ 1.0 0 ~ O O
5-25JJ 5 - - TIO~ 2 - - /~ 50 JJ 1.0 O ~ x O
5-26J~ 5 Nl 3 Si0~ 3 PAS 0.5 11 < 5 11 1.0 O x x O
,~ 5-27 ~ 7 - - ~ 2 ~ 200 ~ 1.0 0 0
5-28 ~ 5 Nl 3 JJ 3 JJ 0.5 ~ 50 ~ 0.1 x x
5-29 ~ 7 ~ 2 )I I ~ 50 ~ 4.0 ~ O x x
5-30JJ - Nl12 -- -- -- -- u 50 JJ 1.0 ~ x O O
~n
~n
-
1 337555
Example 6
Preferred embodiments of electro-plating bath
compositions and electro-plating conditions for obtaining
the coated steel sheets according to the present invention
are shown in Table 6.
Corrosion resistance was evaluated by the area
of red rust after 500 hours salt spray test (JIS Z2371).
less than 1 %: ~
1 % - 10 %: O
10 - 30 %:
more than 30 %: X
Surface brightness was evaluated by brightness
degree (JIS Z8741, Gs (60), standard value of black
glass plate: 93).
brightness degree not less than 50: ~
" not less than 20: O
" less than 20:
turned black: X
Among the coated products obtained by the
20 examples of the present invention, those with the
Cr content of not less than 5 wt.% are particularly
excellent in corrosion resistance. All of the examples
were excellent in the surface brightness.
As for comparison examples, No. 6-27 and 6-29
do not contain cationic polymer in the plating bath and
thereby the current density is not sufficiently high,
so that the Cr content in the coating is only in a trace
amount and the corrosion resistance is not good. While
- 47 -
~ .,;.
1 337555
-
No. 6-28 and 6-30, too, do not contain cationic polymer,
since the current densi~y is sufficiently high to
increase, the Cr content and the corrosion resistance
is excellent, but their surface appearance is inferior.
In the comparison examples No. 6-31 and 6-32, since the
ratio of Cr /Cr3 in the plating bath is high, the
Cr content in the primer coating is low, corrosion
resistance is insufficient and surface appearance, too,
is inferior. By the way, as for the current efficiency,
lO while all the examples of the present invention show
the efficiency of more than 60 %, the efficiency of all
the comparison examples is as low as 50 ~ or lower.
,
; - 48 -
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T a b I e 6 (Cont'd)
Platin~ Condition Coatin~ Co-position (~ei~ht X)
Corrosion Surface
No Current Bath Relative Flo~ Fine Particle Resistance Bri8htness
Density Te-perature VelocityZn Cr Ni,Fe,Co Cation Polyser ûxide
(A/d-2) t C) (o/xin)
i- 115û 6û 9û balance lû - û l - O O
6- 220 ~ 6 - 0 005 -
C- 3100 ~ 7 - 0 05 -
t- 4250 ~ 1O - 0 1 ~
6- 550 ~ 10 - 0 01 -
6- 6200 ~ 6 - 0 5 - ~ O
6- 715û ~1 ~I J~ 10 Ni 3 û l - O O
6- 820 ~ Jl 5 Ni 3 0 005 - O O
Exanple 6
1 6- 9200 ~ / 7 Ni 3 0 05 - O O
6-10250 ~ J 10 Ni 3 0 1 - O O1 6-1150 ~ Jl J~ 10 Ni 3 O ûl - O O
6-12200 11 ~ JJ 5 Ni 3 0 5 - O O
6-13100 ~ 7 - 0 05 -
6-1450 ~ 7 - 0 01 -
6-15150 ~ 7 - 0 1 -
6-16150 ~ 10 - 0 02 -
6-1720 ~ 6 - 0 -
6-18100 ~ 20 - 2 -
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- 52 -
1 337555
Example 7
Continuous electro-plating was done in a plating
bath having the composition as in Example No. 6-1 by
using an anode of Pb-5 % Sn. The ratio of Cr6 /Cr3
in the bath was measured periodically and the current
density was controlled to 150 A/dm2 so as to obtain a
coating of 20 g/m . Until the ratio of Cr6 /Cr3 in
the bath reached to 0.1, the Cr content in the coating
obtained was not less than 6 wt.% and the coating was
excellent in surface brightness, but when said ratio
exceeded 0.1, the Cr content in the coating was decreased
and the surface brightness also deteriorated remarkably
or further the surface was turned to blackO At the time
when the Cr6 /Cr3 ratio in the bath reached to 0.5,
the plating solution was circulated through a bath filled
with Zn metal, and then the Cr6 /Cr3 ratio was decreased.
After the Cr /Cr ratio was lowered to 0.1, the plating
was done again under the above mentioned condition, and
the coating with the Cr content of 7 wt.~ and having
excellent surface brightness could be obtained.
Continuous electro-plating was done thereafter by passing
the plating solution occasionally through the tank filled
with metallic Zn and repeating the measurement of the
Cr6 /Cr3 ratio in the bath periodically. Thus, a coating
with the Cr content of not less than 6 wt.% and excellent
surface brightness could be obtained under the condition
that the Cr /Cr ratio in the bath was not more than
O . 1 .
~_j - 53 -
~ I,;
1 337555
Example 8
With an anode of Pb-5~ Sn and using a plating
bath same as in Example No. 6-7, electro-plating was
done under the condition of Example 7. The result was
the same as in Example 7 until the Cr /Cr ratio did
not exceed 0.1. By adding Fe2 ion in an amount of 5 g/1
to the bath at the time when the Cr6 /Cr3 ratio reached
to 0.5, the Cr6 /Cr3 ratio lowered to 0.05, and a coating
with the Cr content of 10 wt.~ and having excellent
surface brightness could be obtained.
As above described, it is possible according
to the present invention to produce a composite
electro-plated steel sheet having a primer coating
comprising Zn as main constituent, containing
simultaneously a large quantity of Cr and having excellent
surface brightness which has been very difficult to
produce by the conventional arts. The coated products
according to the present invention are very suitable
for the production of a rust preventing steel sheet for
such uses as in automobiles, home electric appliances
and constructions in which high corrosion resistance
and excellent surface brightness are required.
- 54 -
